cancel all that are in common: the ostriches, birds belong to me, and 9 feet tall.

You are left with the conclusion:
The birds in my aviary do not live on mince pies.

Answer^^^

It is made clear that the ostrich is the only bird that is 9 feet tall, thus being the only bird that the person can own but both of which are conclusions that can be plainly seen,along with all the birds in the aviary belonging to the person.
You need to find the one not stated I guess is what I'm trying to say.
Sorry if this is late.